JP4040663B2 - Semiconductor laser and manufacturing method thereof - Google Patents

Description

  The present invention relates to a semiconductor laser and a manufacturing method thereof, and more particularly to a semiconductor laser having a buried structure used for optical fiber communication and a manufacturing method thereof.

  As the application range of optical fiber communication expands from the trunk system of the communication network to the subscriber system, the semiconductor laser that is the light source is required to operate in a wide temperature environment, and in particular, the operating current increases. It is necessary to achieve good laser characteristics at high temperatures. At the same time, requirements are increasing.

  Therefore, there is a need for a structure and manufacturing method that can realize a laser capable of operating up to high temperatures with good uniformity.

  Normally, a buried structure is used in a semiconductor laser used for optical communication. The buried structure constricts current, and there are a structure using a pn junction and a structure using a high resistance layer, but a structure using a pn junction is suitable for high-temperature operation.

  A semiconductor laser having a pn junction buried structure has a structure as shown in FIG.

  In FIG. 1, an active layer 2 made of InGaAsP and a first p-type cladding layer 3 made of p-type (p-) InP are formed on an n-type (n-) InP substrate 1 to form a first p-type. From the mold cladding layer 3 to the upper part of the n-InP substrate 1, a mesa is formed as a mesa portion. The active layer 2 constituting the mesa portion has a stripe shape with a width of about 1 to 1.5 μm, and both sides of the mesa portion are buried regions.

  A p-type buried layer 4 made of p-InP and an n-type current blocking layer 5 made of n-InP are buried in the buried region, and further on the current blocking layer 5 and the first p-type cladding layer 3. The second p-type cladding layer 6 made of p-InP and the p-type contact layer 7 made of p-InGaAs are sequentially formed.

  Further, a p-side electrode 8 is formed on the p-type contact layer 7, and an n-side electrode 9 is formed under the InP substrate 1.

  The semiconductor laser having such a buried structure is obtained by growing an active layer 2 and a first p-type cladding layer 3 on an n-InP substrate 1 and then using the mask to form an n-type semiconductor laser. After etching the upper part of the InP substrate 1 in a substantially stripe shape, a buried structure composed of the p-type buried layer 4 and the n-type current blocking layer 5 is formed on both sides thereof.

  In recent optical communication lasers, a quantum well or a strained quantum well structure is often used for the active layer. The active layer shown below includes a quantum well structure including a well layer and a barrier layer, and a quantum well structure. Including the upper and lower light guide layers sandwiching.

  Specific examples of reports having the structure described above include Kondo et al., 1995 Autumn Applied Physics Society 27p-ZA-5, Chino et al., 1997 Spring Applied Physics Society 30p-NG-11.

  By the way, in a buried semiconductor laser, it is important to reduce a leakage current that does not pass through an active layer in order to obtain good characteristics at a high temperature.

  In the pn buried structure laser illustrated in FIG. 1, both sides of the active layer 2 are buried with the p-type buried layer 4, and this layer is connected to the p-type cladding layers 3 and 6 immediately above the active layer 2. Yes.

  Therefore, during high temperature operation, a leak current is generated that flows from the p-type cladding layers 3 and 6 to the n-type InP substrate 1 through the p-type buried layer 4 through the path indicated by the arrow in FIG. Since the amount of leakage current depends on the distance between the active layer 2 and the n-type block layer 5, in order to reduce the leakage current, the distance between the n-type block layer 5 and the active layer is, for example, about 0.2 μm. It needs to be narrowed. Moreover, in order to obtain uniform laser characteristics, it is necessary to manufacture the distance with good controllability.

  However, in the conventional structure, although the innermost point of the n-type block layer 5 is set at the edge of the mesa top surface, the angle θ of the bottom surface of the n-type block layer 5 close to the active layer 2 is 30 with respect to the horizontal direction. As the distance between the n-type blocking layer 5 and the active layer 2 decreases, the width of the region where the leakage current flows becomes wider.

  The angle of the bottom surface of the n-type block layer 5 depends on the angle of the top surface of the p-type buried layer 4 below. That is, the surface with a gentle angle of about 30 degrees has a slow (111) plane at the beginning of the crystal growth of the p-type buried layer 4, and then the growth rate is related to other plane orientations. The surface that appears when growth starts on the (111) surface, and the position and angle of the surface are between the height of the mesa portion, the shape of the lower portion of the mesa portion, and the plane orientation due to changes in growth conditions. Very sensitive to changes in growth rate relationships, etc.

  Therefore, even if the p-type buried layer 4 is formed using the MOVPE method, which is said to have good controllability, it is difficult to manufacture the position of the n-type block layer 5 with respect to the active layer 2 uniformly with good reproducibility. there were.

  An object of the present invention is to provide a semiconductor laser that can realize a narrow distance between an active layer and a current blocking layer with good controllability in a laser manufactured on a substrate, and a manufacturing method thereof.

As illustrated in FIG. 9, the above-described problem is formed in a stripe shape on the first clad layers 21 and 22 having a mesa-shaped protrusion made of a compound semiconductor containing a first conductivity type impurity, Further, the active layer 23 having a side surface inclined at an angle of 70 degrees or more and 90 degrees or less with respect to the upper surfaces of the first cladding layers 21 and 22 is different from the first conductivity type impurity formed on both sides of the protrusion. One end is in contact with the buried layer 27 containing the second conductivity type impurity and a virtual surface obtained by extending the side surface of the active layer 23 upward, and the first cladding layers 21 and 22 are further directed downward from the one end. A first surface which is a (111) surface inclined at an angle of approximately 55 degrees with respect to the upper surface, and extends downward from the first surface to substantially the same height as the lower surface of the active layer 23 and the active layer 23 On the side of the active layer 23 A second surface that is inclined to be smaller than an angle perpendicular to the upper surface of the first cladding layers 21 and 22 and larger than a side angle, and is formed on and in contact with the buried layer 27; possess a current blocking layer 28a containing a first conductivity type impurity, and a second cladding layer 24, 29 comprising a second conductivity type impurity formed on the current blocking layer 28a and the active layer 23, the The buried layer 27 is formed on the surfaces of the second cladding layers 24 and 29, the side surfaces of the active layer 23, and the side surfaces of the first cladding layers 21 and 22 corresponding to the imaginary surface extending downward from the one end. The current blocking layer 28a is formed in contact with the upper end of the current blocking layer 28a and extends to the side of the active layer 23 and is inclined at an angle of approximately 55 degrees with respect to the upper surfaces of the first cladding layers 21 and 22. (111) this has a surface Solved by a semiconductor laser according to claim.

The semiconductor laser described above further includes a third surface extending in a direction away from the protrusion from a lower end of the second surface .

  In the semiconductor laser described above, the shortest distance between the active layer 23 and the current blocking layer 28 is in the range of 0.1 to 0.3 μm. Alternatively, the current blocking layer 28 has a surface extending from the one end to the side of the active layer 23 and inclined at an angle of about 55 degrees with respect to the surface of the substrate.

As illustrated in FIGS. 8 and 9, the above-described problem is that the active layer 23 and the second cladding layer including the second conductivity type impurity are disposed on the first cladding layers 21 and 22 including the first conductivity type impurity. A step of forming a side layer 24; a step of forming a stripe-shaped mask 25 on the lower layer 24 of the second cladding layer; and a portion of the second cladding layer not covered by the mask 25 A step of dry etching from the side layer 24 to the top of the first cladding layers 21 and 22 to form a planar stripe-shaped mesa portion 26, and extending downward from the edge of the top surface of the mesa portion 26 (111) ) And the angle of the side surface of the active layer 23 on both sides of the active layer 23 and extending from the first surface to substantially the same height as the lower surface of the active layer 23. Perpendicular to the top surface of the first cladding layer A second plane inclined at an angle between the angle, and forming in contact with the buried layer 27 on both sides of the mesa portion 26 which includes a second conductivity type impurity, the buried layer 27 forming a current blocking layer 28 including the first conductivity type impurity having the first surface and in contact on the second surface and extends extends over the edge of the top surface of the mesa portion 26 (111) And after removing the mask 25, an upper layer 29a of the second cladding layer containing a second conductivity type impurity is formed on the lower layer 24 of the second cladding layer and the current blocking layer 28a. And a process for manufacturing the semiconductor laser.

  In the semiconductor laser manufacturing method described above, the buried layer 27 has a substantially zero growth rate on both side surfaces after a predetermined film thickness is formed on both side surfaces of the active layer 23. As described above, it is formed through a growth process in which the growth locally proceeds only below the active layer. In this case, a gas containing methyl chloride is used in the growth process of the buried layer, and the growth rate on the both side surfaces of the active layer is changed to substantially zero by increasing the flow rate of the methyl chloride gas. There is a method in which the growth is locally advanced only below the active layer.

  In the semiconductor laser manufacturing method described above, the buried layer 27 is grown using a chlorine-containing gas together with a source gas. In this case, the chlorine-containing gas includes methyl chloride gas.

  In addition, the above-mentioned figure number and code | symbol are quoted in order to make an understanding of invention easy, and are not limited to this.

  Next, the operation of the above invention will be described.

  According to another aspect of the present invention, the angle of the side surface of the active layer formed on the mesa-shaped first cladding layer is set within a range of 70 to 90 degrees with respect to the upper surface of the first cladding layer. One end of the current blocking layer is brought into contact with the upward extension of the side surface, and the angle of the surface of the current blocking layer extending from the one end to the lower side of the active layer is substantially inclined by 55 degrees.

  This narrows the buried layer under the current blocking layer on both sides of the active layer, so that a leakage current passing region from the second cladding layer over the active layer through the buried layer can be obtained. It becomes narrow and the leakage current is reduced. As a result, the current-light output characteristics at high temperatures and high outputs can be made uniform.

  In such a semiconductor laser manufacturing method, an active layer and a lower layer portion of a second cladding layer are sequentially formed on the first cladding layer, and then the upper layer portion of the first cladding layer is formed from the lower layer portion of the second cladding layer. After forming the mesa portion by patterning using dry etching, the growth of the buried layer is controlled so that the (111) plane exists from the side to the lower side of the active layer. This is achieved by forming a current blocking layer.

  In this case, the (111) plane of the buried layer has an inclination of approximately 55 degrees with respect to the substrate surface. Also, by leaving a surface of the buried layer below the (111) plane that is substantially parallel to the side surface of the mesa, it is possible to form a layer of another surface on the (111) plane. Can be prevented.

  According to the invention, the angle of the side surface of the active layer formed on the mesa-shaped first cladding layer is set within a range of 70 to 90 degrees with respect to the upper surface of the first cladding layer, and One end of the current blocking layer is brought into contact with an extension extending upward from the side surface, and the angle of the surface of the current blocking layer extending downward from the one end is substantially inclined by 55 degrees, and the side surface of the active layer is formed on both sides of the active layer. It is inclined so as to be larger than this angle and smaller than 90 degrees.

  As a result, the distance between the active layer and the current blocking layer is the shortest along both side surfaces of the active layer, so that the region of the buried layer between them becomes narrower, and the leakage current passing through the region further increases. To reduce.

  In such a semiconductor laser manufacturing method, an active layer and a lower layer portion of a second cladding layer are sequentially formed on the first cladding layer, and then the upper layer portion of the first cladding layer is formed from the lower layer portion of the second cladding layer. After patterning using dry etching to form the mesa portion, the (111) plane exists up to the upper side of the active layer, and the buried layer is formed under such a condition that a plane substantially parallel to the active layer appears below it. This is achieved by controlling growth.

  In this case, it is difficult to turn on the parasitic thyristor by reducing the thickness of the buried layer grown on the side surface of the active layer and increasing the thickness of the buried layer below the active layer and the current blocking layer. There is a need to. For this purpose, there is a method in which a chlorine-containing gas is used during the growth of the buried layer, and a method of increasing the amount of introduction of the chlorine-containing gas after forming a desired film thickness on the side surface of the active layer. In this way, the film thickness grown on the substrate surface can be locally increased.

  As described above, by using the present invention, it becomes possible to fabricate the buried layer in the region where the leakage current that affects the characteristic variation at high temperature flows with good reproducibility. A great contribution to uniform characterization.

  As described above, according to another embodiment of the present invention, the angle of the side surface of the active layer formed on the mesa-shaped first cladding layer is in the range of 70 to 90 degrees with respect to the upper surface of the first cladding layer. In addition, one end of the current blocking layer is brought into contact with the upper extension of the side surface, and the angle of the surface of the current blocking layer extending from the one end to the lower side of the active layer is substantially inclined by 55 degrees. I am doing so. Therefore, since the buried layers existing on both sides of the active layer are narrowed, the leakage current flowing from the second cladding layer above the active layer to the buried layer is narrowed to reduce the leak current. The current-light output characteristics at high temperature and high output can be made uniform.

  According to the invention, the angle of the side surface of the active layer formed on the mesa-shaped first cladding layer is set within a range of 70 to 90 degrees with respect to the upper surface of the first cladding layer, and One end of the current blocking layer is brought into contact with the upward extension of the side surface, and the angle of the surface of the current blocking layer extending downward from the one end is substantially inclined by 55 degrees, and the active layer is formed on both sides of the active layer. It is larger than the angle of the side surface and smaller than 90 degrees. Therefore, since the distance between the active layer and the current blocking layer is the shortest along both side surfaces of the active layer, the buried layer region between them is narrowed, and the leakage current passing through the region is further reduced. can do.

  Accordingly, embodiments of the present invention will be described below with reference to the drawings.

  2 and 3 show the steps until the buried layer of the semiconductor laser according to the embodiment of the present invention is formed.

  First, as shown in FIG. 2 (a), an n-type buffer layer (n-type cladding layer) 22 having a film thickness of 300 to 1000 nm is formed on the (100) surface of the n-InP substrate 21 by a MOVPE method, and a film thickness of 200 to An MQW (multi-quantum well) active layer 23 made of 300 nm undoped InGaAsP and a first p-type cladding layer 24 made of p-InP with a thickness of 250 to 700 nm are formed in this order.

InP is grown using trimethylindium (TMIn) and phosphine (PH ₃ ) as source gases, and InGaAsP is grown using TMIn, PH ₃ , arsine (AsH ₃ ), and triethylgallium (TEGa) as source gases. Further, dimethyl zinc (DMZn) is used as the p-type dopant, and silane (SiH ₄ ) is used as the n-type dopant.

  The MQW active layer 23 is a barrier formed between, for example, a five-period well layer made of 6 nm-thick InGaAsP with a compressibility of 1% and a 10 nm-thick InGaAsP lattice-matched with InP. And a light guide layer made of InGaAsP with a thickness of 100 nm formed above and below the well layer and the barrier layer. Each of the barrier layer and the light guide layer is made of InGaAsP having a composition in which the wavelength corresponding to the band gap is 1.1 μm. Thereby, a laser having a wavelength band of 1.3 μm is formed.

  The layer structure of the MQW active layer 23 is not limited to this, and may be composed of an InGaAsP system in the 1.55 μm band, 1.48 μm band, and other wavelength bands.

The n-InP buffer layer 22 has an impurity concentration of about 5 × 10 ¹⁷ atoms / cm ³ , and the p-InP cladding layer 24 has an impurity concentration of about 5 × 10 ¹⁷ atoms / cm ³ .

  Note that the n-type buffer layer 22 made of n-InP under the active layer 23 and the n-InP substrate 21 function as an n-type cladding layer.

After finishing the first film growth as described above, a dielectric film, for example, a SiO ₂ film is formed on the first p-type cladding layer 24 to a thickness of about 0.3 μm by the CVD method. Then, the dielectric film is patterned into a stripe shape having a width of about 1.5 μm elongated in the <011> direction by photolithography, and this is used as the mask 25.

Next, as shown in FIG. 2 (b), the portion from the p-InP cladding layer 24 not covered by the mask 25 to the upper portion of the n-InP substrate 21 is etched to a depth of about 2 to 3 μm. A mesa portion 26 having a mesa-shaped cross-section and a stripe-shaped projection is formed below 25. The etching is performed by a reactive ion etching (RIE) method, and an ethane-based gas, for example, a mixed gas of C ₂ H ₆ , O _2, and H ₂ is used.

  Thereafter, the surface of each compound semiconductor layer damaged by dry etching is removed by a thickness of about 0.1 μm by an acid treatment such as sulfuric acid, and then a p-InP buried layer 27 is grown in the concave portions on both sides of the mesa portion 26. To do.

  The p-InP buried layer 27 is grown through the following process.

  First, the mask 25 is in a state in which it does not protrude from the top surface of the mesa portion 26 in a bowl shape, and the side surface of the mesa portion 26 is in a state of being inclined steeply by 70 degrees or more with respect to the horizontal direction (that is, the substrate surface). It has become.

In such a state, the p-InP buried layer 27 is formed by the second MOVPE method. When the p-InP buried layer 27 is grown, TMIn and PH ₃ are introduced into the reaction chamber of the MOVPE apparatus as source gases, and methyl chloride (CH ₃ Cl) is introduced into the reaction chamber together with the source gas. By preventing the growth of InP on the mask 25, the (111) plane grows from the edge of the top surface of the mesa portion.

  That is, as shown in FIG. 3A, with respect to the p-InP buried layer 27, the (111) plane appears downward from the edge of the top surface of the mesa portion, and on the side surface of the mesa portion 26, the side surface is almost on the side surface. A parallel surface appears, a surface with a gentle angle of about 30 degrees with respect to the substrate surface appears in the vicinity of the bottom of the mesa portion 26, and (100) surfaces appear on the surface of the substrate on both sides of the mesa portion 26.

  Further, as the growth of the p-InP buried layer 27 proceeds, as shown in FIG. 3B, the (111) plane extends obliquely downward and further expands, but is substantially parallel to the side surface of the mesa portion 26. The p-InP surface becomes shorter along its side. Further, the p-InP surface substantially parallel to the side surface of the mesa portion 26 gradually approaches the substrate surface little by little as the growth proceeds.

  When the growth of the p-InP buried layer 27 further proceeds, the p-InP plane substantially parallel to the side surface of the mesa portion 26 of the buried layer 27 disappears as shown in FIG.

  Further, when the growth of the p-InP buried layer 27 progresses, as shown in FIGS. 4A and 4B, a gentle angle of about 30 degrees growing from the bottom of the mesa portion 26 in the buried layer 27 is obtained. Since the surface also grows on the (111) plane, the (111) plane becomes shorter with the growth. The appearance of this shape depends on the growth rate in each plane orientation.

  In the growth process of the p-InP buried layer 27 described above, in the region (corner portion) where the angle of the surface changes, a surface and a transition region not described above are generated, but are omitted in the drawing.

  By utilizing the difference in shape due to the growth process of the p-InP buried layer 27 as described above, a semiconductor laser having a structure as described below can be formed. In the following three examples of semiconductor lasers, the process up to the formation of the mesa portion 26 is omitted.

First Example The semiconductor laser described below stops the growth of the buried layer 27 when the plane parallel to the side surface of the mesa portion 26 of the surface of the buried layer 27 disappears, and then the current The example which forms a block layer is shown.

  That is, as shown in FIG. 5A, the (111) plane, the 30-degree inclined plane, and (100) in the direction from the edge of the top surface of the mesa portion 26 to the bottom of the buried layer 27 that has been grown. The surfaces are exposed in order.

  In this case, the thickness of the active layer 23 is about 0.3 μm, the thickness of the first p-type cladding layer 24 is 0.4 μm, the height of the mesa portion 26 is about 2 μm, and the active portion of the mesa portion 26 is The angle of the side surface of the layer 23 is about 83 degrees. Further, the film thickness at the flat portion of the p-InP buried layer 27 is 0.7 μm, and the angle of the (111) plane of the p-Inp buried layer 27 is about 55 degrees with respect to the horizontal line. It has become.

  From this state, as shown in FIG. 5B, when an n-InP current blocking layer 28 is grown on the p-Inp buried layer 27 on both sides of the mesa portion 26, the n-InP current blocking layer 28 is grown. Has the same shape as the upper surface of the p-InP buried layer 27. In addition, one end of the n-InP current blocking layer 28 is positioned 0.4 μm above the active layer 23 on the extension of both side surfaces of the mesa portion 26.

  In this case, the thickness of the p-InP buried layer 27 at the lower end of the side surface of the active layer 24 is about 0.4 μm, and the shortest distance from the upper end of the active layer 24 to the n-InP current blocking layer 28 is about 0. 19 μm. However, the thickness of the first p-type cladding layer 24 and the inclination of the side surface of the mesa portion 26 may be adjusted so that the shortest distance is in the range of 0.1 to 0.3 μm.

  When the film thickness in the flat region of the n-InP current blocking layer 28 is about 0.9 μm, the bottom surface of the n-InP current blocking layer 28 is bordered on one end closest to the first p-type cladding layer 24. As described above, the (111) plane, which is the boundary surface with the p-InP buried layer 27, extends below the active layer 24, and the upper surface thereof is first the (111) plane as it moves away from one end thereof. An ascending slope appears, followed by a flat surface and a descending slope.

  After completing the growth of the n-InP current blocking layer 28 and removing the mask 25, the process proceeds to the third crystal growth step.

  In the third MOVPE crystal growth, as shown in FIG. 5 (c), a first p-InP film having a thickness of about 1.5 μm is formed on the n-current blocking layer 28 and the first p-InP cladding layer 24. Two p-type cladding layers 29 are formed. Further, an intermediate layer 30 made of p-InGaAsP having a thickness of 0.2 μm and a p + -type InGaAs having a thickness of 0.5 μm are formed on the second p-type cladding layer 29. A contact layer 31 is formed.

  Next, as shown in FIG. 6, after forming a p-side electrode 32 made of Ti / Pt / Au on the contact layer 31, an n-side electrode 33 made of AuGe / Au is formed on the lower surface of the n-InP substrate 21. Form.

  The film thickness described above is a value in a flat region unless otherwise specified.

  This completes the basic structure of the semiconductor laser.

  In the semiconductor laser as described above, since the surface of the n-InP current blocking layer 28 close to the mesa portion 26 is at an angle of 55 degrees with respect to the horizontal plane, the n-InP current blocking layer 28 is the active layer. Approaching at a distance of 2 μm or less.

  Therefore, the region where the first and second p-InP cladding layers 24 and 29 and the p-InP buried layer 27 are connected is narrower than the conventional one, and the leakage current passing through the region is reduced.

  Further, since the growth conditions and the like of the buried layer 27 are set so that the (111) plane appearing in the vicinity of the active layer 23 in the p-InP buried layer 27 extends below the active layer 23, the mesa portion 26 The (111) plane is always located on both sides of the active layer 23 even if the height of the layer slightly changes due to manufacturing errors.

  Therefore, the distance between the n-InP current blocking layer 28 and the active layer 23 is substantially determined by the distance from the active layer 23 to the top surface of the mesa portion 26 (that is, the first p-type cladding layer 24). The manufacturing error in the distance between the n-InP current blocking layer 28 and the n-InP current blocking layer 28 depends only on the angle deviation of the side surface of the mesa portion 26. Therefore, the shortest width of the region through which the leakage current passes, that is, the shortest distance between the active layer 23 and the current blocking layer 28 does not depend on the film growth conditions and the manufacturing deviation of the height of the mesa portion 26. Can be stably reduced, current-light output characteristics at high temperatures and high outputs can be made uniform, and reproducibility can be improved.

  In this example, the upper (111) plane of the n-InP current blocking layer 28 is inclined at an angle of about 55 degrees, and the uppermost portion of the (111) plane is higher than the mesa portion 26. Thus, when the upper part of the current block layer 28 on both sides of the mesa portion 26 is tapered so as to become narrower toward the active layer 23, the thickness of the current block layer 28 increases rapidly in the vicinity of the active layer 23. As a result, the injection current can be effectively collected on the active layer 23.

  However, the upper portion of the current blocking layer 28 is not necessarily raised on both sides of the mesa portion 26.

  Further, as described above, since the side surface of the mesa portion 26 is formed in a shape that is almost perpendicular to about 83 degrees, the n-InP layer (n-InP buffer layer 22, n-InP substrate below the active layer 23). 21) and the n-InP current blocking layer 28 have a structure in which the distance gradually increases downward. Therefore, the first and second p-type cladding layers 24 and 29 made of p-InP, the n-InP current blocking layer 28, the p-InP buried layer 27, and the n-InP layer below the mesa portion 26 (n− A pnpn thyristor comprising the InP buffer layer 22 and the n-InP substrate 21) is difficult to turn on.

  Therefore, considering only the control of the shortest distance between the active layer 23 and the current blocking layer 28 as described above, even with a gently flared shape such that the lower part of the active layer 23 is formed by wet etching. The shortest distance can be controlled. However, from the viewpoint of the current blocking characteristics of the thyristor, it can be said that the mesa portion 26 having a shape having a nearly vertical side surface formed by dry etching as in this example is preferable.

Second Example In this example, the height of the mesa portion 26 is set to a high value of about 2.5 μm, and the growth of the p-InP buried layer 27 is completed, and the surface of the p− becomes substantially parallel to the mesa. The structure is characterized in that the InP surface remains in the buried layer 27. That is, in this example, the growth of the p-InP buried layer is stopped at the stage shown in FIG.

After the p-InP buried layer 27 is formed, the n-InP current blocking layer 28 is formed on the p-InP buried layer 27 and the mask 25 is removed as in the first example. After that, a second p-type cladding layer 29, a p-InGaAsP intermediate layer 30 and a contact layer 31 made of p ⁺ -type InGaAs are formed on the n-InP current blocking layer 28 and the first p-InP cladding layer 24 by the MOVPE method. Formed by. Thereafter, by forming the p-side electrode 32 and the n-side electrode 33, the semiconductor laser having the structure shown in FIG. 7 is obtained.

  As described above, if the p-InP plane parallel to the side surface of the mesa portion 26 is planned to be left in a part of the p-InP buried layer 27 before the p-InP buried layer 27 is grown, Even if the growth rate of the p-InP buried layer 27 slightly changes due to a change in some condition, p-InP does not grow on the (111) plane of the buried layer 27, and the p-InP on the active layer 23 is lost. The film thickness control, that is, the control of the distance between the n-type current blocking layer 28 and the active layer 23 becomes easier and more reliable.

Third Example In the first example described above, the growth of the p-InP buried layer 27 is stopped in the state between FIG. 3C and FIG. 4A, and then the n-InP current blocking layer 28 is formed. Formed.

  On the other hand, in this example, a process of stopping the growth of the p-InP buried layer 27 in a state close to that shown in FIG. 3B and then growing an n-InP current blocking layer or the like is employed.

  That is, as shown in FIG. 8A, the surface shape of the buried layer 27 after the growth has a (111) plane obliquely downward from the edge of the top surface of the mesa portion 26, and both sides of the active layer 23. A plane substantially parallel to the side surface of the mesa portion 26 appears on the side, and a 30-degree inclined surface and a (100) plane appear in turn below the active layer 23. In this case, the surface substantially parallel to the active layer 23 of the upper surface of the buried layer 27 is a surface that is larger than the inclination of the side surface of the active layer 23 and smaller than the angle perpendicular to the substrate surface.

  In this case, the height of the mesa portion 26 is about 2 μm, the thickness of the active layer 23 in the mesa portion 26 is about 0.3 μm, and the lower end of the active layer 23 is about 1.3 μm from the bottom of the mesa portion 26. In the position. Further, the film thickness in the flat portion of the p-InP buried layer 27 is 0.6 μm, and the angle of the (111) plane is about 55 degrees with respect to the substrate surface (horizontal plane). Further, the film thickness of the p-InP buried layer 27 on the side surface of the active layer 23 constituting the mesa portion 26 is about 0.2 μm.

  When growing the p-InP buried layer 27 having such a shape, the following conditions are required.

  First, it is desirable that the mesa portion 26 is formed by dry etching and has a side surface that is nearly perpendicular to the substrate surface. This is because, since film growth on the surface close to the (211) plane proceeds rapidly, p-InP is obtained when a mesa portion is formed by wet etching and a gentle inclined surface as shown in FIG. This is because a surface substantially parallel to the side surface of the mesa portion of the buried layer 27 disappears quickly.

  Further, since the p-InP buried layer 27 is grown only on the side surface of the active layer 23 with a film thickness of about 0.2 μm, the simple method of using only the source gas is on the horizontal plane (on the substrate surface). The p-InP buried layer 27 cannot be formed thick.

  If the thickness of the flat portion of the p-InP buried layer 27 is reduced on the n-InP substrate 21, the pnpn thyristor formed on both sides of the mesa portion 26 is easily turned on, and the leakage current increases. End up. Therefore, the p-InP buried layer 27 having this structure has a thickness of 0.2 μm on the side surface of the active layer 23 and a thickness of 0.6 μm on the horizontal plane of the n-InP substrate 21. Is adopted.

In the same way as in the first example, a small amount of CH ₃ Cl (partial pressure is about 14 mTorr) is introduced into the reaction chamber in addition to TMIn and PH ₃ as in the first example. A p-InP layer having a thickness of about 0.2 μm is first grown, and subsequently, the amount of CH ₃ Cl introduced is increased to a partial pressure of about 92 mTorr to grow p-InP having a thickness of about 0.4 μm.

When the CH ₃ Cl flow rate is increased during the growth of p-InP constituting the buried layer 27, the growth rate on the side surface of the mesa portion 26 rapidly decreases with respect to the growth rate on the substrate surface. Eventually, almost no growth occurs on the side surface of the mesa portion 26, and only the p-InP thickness on the substrate surface increases. According to such a method, the p-InP buried layer 27 having a shape as shown in FIG. 8A is obtained, and a pnpn thyristor structure is obtained in which the p-InP layer is thin only on the side surface of the mesa portion 26.

  In this case, the lower end of the surface of the p-InP buried layer 27 that is substantially parallel to the side surface of the mesa portion 26 is positioned substantially on the extension of the lower surface of the active layer 23.

  After the p-InP buried layer 27 is formed as described above, an n-InP current blocking layer 28a is formed on the p-InP buried layer 27 on both sides of the mesa portion 26 as shown in FIG. grow up. The shape of the bottom surface of the n-InP current blocking layer 28a is the same as the top surface of the p-InP buried layer. In this case, as in the first example, the n-InP current blocking layer 28a has one end in contact with the edge of the top surface of the first p-type cladding layer 24, and obliquely upward and obliquely downward from the one end. Make the (111) plane appear.

  As a result, the n-InP current blocking layer 28a in the region facing the side surface of the active layer 23 has a structure that is approximately parallel to the side surface of the active layer 23 and separated by 0.2 μm.

  With such a structure, the leakage current flowing from the n-InP current blocking layer 28a through the narrow p-InP buried layer 27 into the n-InP layer below the active layer 23 is reduced.

  After completing the growth of the n-InP current blocking layer 28a and removing the mask 25, the process proceeds to the third crystal growth step.

In the third crystal growth, as shown in FIG. 8C, a second p-InP having a film thickness of about 1.5 μm is formed on the n-InP current blocking layer 28a and the first p-InP cladding layer 24. A clad layer 29a is formed, and an intermediate layer 30 made of p-InGaAsP having a thickness of 0.2 μm and a contact layer made of p ⁺ -type InGaAs having a thickness of 0.5 μm are formed on the second p-InP clad layer 29a. 31 is formed by the MOVPE method.

  Next, as shown in FIG. 9, after forming a p-side electrode 32 made of Ti / Pt / Au on the contact layer 31, an n-side electrode 33 made of AuGe / Au is formed on the lower surface of the n-InP substrate 21. Form.

  This completes the basic structure of the semiconductor laser.

  The semiconductor laser as described above has a structure in which a part of the surface of the p-InP buried layer 27 on both sides of the active layer 23 is substantially parallel to the side surface of the mesa portion 26 (active layer 23). .

  Therefore, in this structure, the shortest distance between the active layer 23 and the current blocking layer 28a is not the shortest point in the side surface of the active layer 23 as shown in FIG. Therefore, the effect of reducing the leakage current is greater than that of the semiconductor laser of the first example, and the current-light output characteristics at high temperature and high output are uniform. However, unlike the first example, the distance between the n-InP current blocking layer 28 a and the active layer 23 is not automatically determined and is controlled by the growth thickness of the p-InP buried layer 27.

  However, the controllability of the growth rate on a specific surface formed before the growth of the p-InP buried layer 27 is newly achieved during the growth as a result of the difference in the growth rate depending on the plane orientation as in the conventional example. This is much better than the position control of the appearing surface.

  That is, the side surface of the mesa portion 26 is determined before the formation of the p-InP buried layer 27, and the film growth control on the side surface is compared with the film growth control on the completely flat substrate surface. Although it is inferior, it has a controllability of about 0.01 μm, and the uniformity of the film thickness is remarkably improved as compared with the conventional case.

  In this example, the mesa angle dependency of the growth rate of the p-InP buried layer 27 is almost in the range of several degrees, and some fluctuation of the mesa angle is absorbed by the growth of the buried layer 27. Is done.

  In the above three examples, a Fabry-Perot type semiconductor laser has been described. However, the present invention is similar to a DFB laser or DBR laser having a diffraction grating, a narrow emission angle laser integrated with a tapered waveguide, or a semiconductor optical amplifier. Of course, the present invention can also be applied to an optical element having a buried structure.

FIG. 1 is a cross-sectional view showing a conventional semiconductor laser. FIGS. 2A and 2B are sectional views (No. 1) showing structural differences due to the growth process of the buried layer used in the semiconductor laser of the embodiment of the present invention. FIG. 3A to FIG. 3C are cross-sectional views (part 2) showing structural differences due to the growth process of the buried layer used in the semiconductor laser of the embodiment of the present invention. FIGS. 4A and 4B are cross-sectional views (part 3) showing structural differences due to the growth process of the buried layer used in the semiconductor laser according to the embodiment of the present invention. FIG. 5A to FIG. 5C are cross-sectional views showing the manufacturing steps of the first example of the semiconductor laser according to the embodiment of the present invention. FIG. 6 is a cross-sectional view showing a first example of a semiconductor laser according to an embodiment of the present invention. FIG. 7 is a sectional view showing a second example of the semiconductor laser according to the embodiment of the present invention. FIG. 8A to FIG. 8C are cross-sectional views showing a manufacturing process of the third example of the semiconductor laser according to the embodiment of the present invention. FIG. 9 is a sectional view showing a third example of the semiconductor laser according to the embodiment of the present invention.

Explanation of symbols

  21 ... n-InP substrate, 22 ... n-InP buffer layer, 23 ... active layer, 24 ... p-InP first p-type cladding layer, 25 ... mask, 26 ... mesa portion, 27 ... buried layer, 28, 28a ... n-InP current blocking layer, 29, 29a ... p-InP second p-type cladding layer, 31 ... contact layer.

Claims (8)

A first cladding layer made of a compound semiconductor containing a first conductivity type impurity and having mesa-shaped protrusions;
An active layer having a side surface formed in a stripe shape on the protrusion and inclined at an angle of 70 degrees to 90 degrees with respect to the upper surface of the first cladding layer;
A buried layer including a second conductivity type impurity different from the first conductivity type impurity formed on both sides of the protrusion;
The active layer side to contact with one end on the virtual plane obtained by extending the upper side of the further inclined at an angle of approximately 55 degrees relative to said top surface of said first cladding layer downward from the one end (111) plane A first surface, and extending downward from the first surface to substantially the same height as the lower surface of the active layer, and at a side of the active layer, the first cladding layer being larger than an angle of a side surface of the active layer A current blocking layer containing a first conductivity type impurity formed on and in contact with the buried layer and having a second surface inclined at an angle smaller than an angle perpendicular to the upper surface of
Have a second cladding layer comprising a second conductivity type impurity formed on said current blocking layer and the active layer,
The buried layer is formed in contact with a surface of the second cladding layer corresponding to the virtual surface extending downward from the one end, a side surface of the active layer, and a side surface of the first cladding layer,
Above the one end of the current blocking layer, there is a (111) plane that extends to the side of the active layer and is inclined at an angle of approximately 55 degrees with respect to the top surface of the first cladding layer. A semiconductor laser characterized by the above. 2. The semiconductor laser according to claim 1, further comprising a third surface extending in a direction away from the protrusion from a lower end of the second surface.   3. The semiconductor laser according to claim 1, wherein a shortest distance between the active layer and the current blocking layer is in a range of 0.1 to 0.3 μm. Forming an active layer and a lower layer of the second cladding layer containing the second conductivity type impurity on the first cladding layer containing the first conductivity type impurity;
Forming a stripe-shaped mask on the lower layer of the second cladding layer;
A step of dry etching from a lower layer of the second cladding layer to an upper portion of the first cladding layer in a portion not covered with the mask to form a planar stripe-shaped mesa portion;
A first surface which is a (111) surface extending downward from the edge of the top surface of the mesa portion, and extends downward from the first surface to substantially the same height as the lower surface of the active layer, and the active layer Embedded on the opposite sides of the active layer and having a second surface inclined at an angle between the angle of the side surface of the active layer and the angle perpendicular to the upper surface of the first cladding layer, and containing a second conductivity type impurity Forming a layer in contact with both side surfaces of the mesa portion;
A current block including a first conductivity type impurity having a (111) plane in contact with the first surface and the second surface of the buried layer and extending upward from an edge of the top surface of the mesa portion Forming a layer;
Forming an upper layer of the second cladding layer containing a second conductivity type impurity on the lower layer of the second cladding layer and the current blocking layer after removing the mask. A method for manufacturing a semiconductor laser. The buried layer, after forming a predetermined film thickness on both sides of the active layer, further grows locally only on the lower side of the active layer with the growth rate on both sides being substantially zero. 5. The method of manufacturing a semiconductor laser according to claim 4 , wherein the semiconductor laser is formed through a growth process proceeding to step (5) . In the growth process of the buried layer, a gas containing methyl chloride is used, and by increasing the flow rate of the methyl chloride gas, the growth rate on the both side surfaces of the active layer is changed to substantially zero and the activity is increased. 6. The method of manufacturing a semiconductor laser according to claim 5 , wherein the growth is locally advanced only below the layer. 5. The method of manufacturing a semiconductor laser according to claim 4 , wherein the buried layer is grown using a chlorine-containing gas together with a source gas. 8. The method of manufacturing a semiconductor laser according to claim 7 , wherein the chlorine-containing gas is methyl chloride gas.

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